Merge branch 'issue_2' into 'master'

Issue 2 (Proper data distribution with Horvod) +  Licensing

Closes #2

See merge request !2

horovod/keras/mnist.py

+# Copyright (c) 2019 Forschungszentrum Juelich GmbH.
+# This code is licensed under MIT license (see the LICENSE file for details).
+# This code is derived from https://github.com/horovod/horovod/blob/master/examples/keras_mnist.py,
+# which is licensed under the Apache License, Version 2.0 (see the NOTICE file for details).
+
 from __future__ import print_function
 import os
 import sys